Fabrication of resonant tunneling transistor lasers for optoelectronic integrated circuits

Date of Completion

January 1997


Engineering, Electronics and Electrical




Semiconductor laser technology has evolved tremendously during the last decade and there have been relentless efforts toward improved performance. The major part of optoelectronic research has focused on III-V material structures for discrete devices such as detectors, lasers, modulators, high-speed field effect transistors (FETs) and heterojunction bipolar transistors (HBT's). There is simultaneous focus on monolithic integration of the various devices to produce optoelectronic integrated circuits (OEIC's) with greater functionality and speed. A laser diode integrated with heterojunction bipolar transistors (HBTs) can be controlled either electronically or optically.^ This study involves the fabrication of a Resonant Tunneling Transistor Laser, a bistable light emitting device which can be triggered electrically or optically. The structure of the RTTL is made up of a HBT with a resonant tunneling structure (RTS) located in the emitter. The electrical characteristics of RTT have been simulated according to a model developed by LaComb et al., for generic resonant tunneling transistors. The overall characteristics of the RTTL were obtained by dividing it into a resonant tunneling diode (RTD) and a HBT structure connected in series.^ The optical cavity which is formed by the waveguiding structure is designed to lase when the base current density is greater than the threshold current density. The p-base which is the active (light emitting) layer, is made of GaAs whose index of refraction is higher than the surrounding AlGaAs layers thus forming a waveguiding structure. A unique feature of this device is the indirect contacting of the active layer which requires p-type zinc diffusion performed through the windows opened in the SiO$\sb2$ isolation layer. Simulation of p-type diffusion is in close agreement with the experimental measurements. Since the diffusion profile is highly non-linear, we calibrated the process using a ZnAs$\sb2$ source in a diffusion furnace at 700$\sp\circ$C adopting a closed ampoule technique. Low resistance ohmic contacts were obtained using Ti/Pt/Au and Au-Ge/Ni for p and n contacts respectively.^ Results obtained from a light-emitting AlGaAs-GaAs heterojunction bipolar device will be presented. The I-V characteristics of the completely processed samples show the negative differential resistance (NDR) behavior in the samples that have been mesa-etched to expose the RTS in the emitter. The transistor characteristics yield a $\beta$ of $\sim$7-15 which matches the simulated characteristics predicted by the RTT model. Bistability was more pronounced at 77 K, and the transistor current gain doubled, yielding a maximum $\beta$ of $\sim$35. The device was tested for the lasing characteristics under pulsed conditions, which resulted in an emission spectrum centered at 906nm. Several devices tested (in excess of twenty) yielded similar results. ^